For the purposes of this application, the definition of "lens" is intended to include a variety of transparent plastic articles which permit viewing, including but not limited to vision correcting ophthalmic eyeglass, sunglass lenses and safety lenses which are typically mounted in a frame as spectacles; piano safety lenses and sunglass lenses without optical power needed for vision correction; shield lenses for sunwear and sports optics products, typically of piano configuration; and visors.
Dielectric coatings are commonly applied to plastic lenses to achieve a variety of optical effects. Two of the most common types of optical coatings are antireflection coatings ("AR coatings") which are employed on ophthalmic eyeglass lenses, and colored mirror coatings which are employed on sunglass lenses and sports optics. In the case of AR coatings, a multilayered coating structure composed of alternating layers of dielectric material(s) with relatively high refractive index, and dielectric material(s) with relatively lower refractive index is deposited onto the convex and concave sides of a lens. The thickness and number, and type of individual coating layers, is chosen using techniques that are well-known in the prior art, based on the refractive index of each dielectric material and the desired reflection spectrum of the complete coating. Multilayered coating structures having alternating layers of low refractive index material and high refractive index material, with a low refractive index material as the top coating layer are the most common broad-band AR coatings used on plastic lenses today. The most common dielectric materials used include a variety of optically transparent oxides, nitrides, and fluorides.
Typical dielectric materials used in optical coatings (and their refractive indices, n, at 550 nm) include, but are not limited to: aluminum oxide (Al.sub.2 O.sub.3,n=1.63), barium fluoride (BaF.sub.2, n=1.47), boron nitride (BN, n=1.8-2.3), hafnium oxide (HfO.sub.2, n=2.02), lanthanum fluoride (LaF.sub.3, n=1.58), lanthanum oxide (La.sub.2 O.sub.3, n=1.90), magnesium fluoride (MgF.sub.2, n=1.38), magnesium oxide (MgO, n=1.70), scandium oxide (Sc.sub.2 O.sub.3, n=1.90), silicon monoxide (SiO, n=2.00), silicon dioxide (SiO.sub.2, n=1.46), silicon nitride (Si.sub.3 N.sub.4, n=2.10), silicon oxy-nitride (SiO.sub.x N.sub.y, n=1.7-2.0), tantalum oxide (Ta.sub.2 O.sub.5, n=16), titanium oxide (TiO.sub.2, n=2.32), tin oxide (SnO.sub.2, n=2.00), indium tin oxide (InSnOx, n=2.02), yttrium oxide (Y.sub.2 O.sub.3, n=1.88), zinc oxide (ZnO, n=2.10), zinc selenide (ZnSe, n=2.65), zinc sulfide (ZnS, n=2.36), zirconium oxide (ZrO.sub.2, n=2.05), and so-called "Substance 1", which is a mixture of zirconium oxide and zirconium titanate (ZrO.sub.2 +ZrTiO.sub.4, n=2.10). Diamond-like carbon (DLC), with refractive index controllable between 1.7 and 2.2, could also be utilized as a high index material in an AR coating stack.
In the case of reflective colored mirror coatings, e.g. on sunglass lenses or sports optics, a multilayered coating structure, typically composed either of alternating layers of high refractive index dielectric material(s) and low refractive index dielectric materials(s), or a single high refractive index layer is deposited onto the convex side of a lens. A layer of silicon monoxide, silicon dioxide, or other material may be used as an adhesion layer between the substrate and the high refractive index material. Finally, to enhance the amount of reflectivity in the mirror coating, a thin metallic layer may be incorporated as one of the layers in the coating stack. In this case, the thickness of the metallic layer is typically &lt;100 .ANG.. Typical metallic materials used as reflecting layers in optical coatings include, but are not limited to: aluminum, chromium, germanium, hafnium, silicon, tantalum, titanium, and zirconium. Application of one or more transparent dielectric layers on top of the thin metallic layer produces a reflected color by optical interference effects which are well known in the art. A variety of dielectric materials, including zirconium oxide, titanium oxide, hafnium oxide, silicon dioxide, silicon monoxide and DLC are used today in the commercial production of colored mirror coatings for sunwear products and sports optics, including lenses and wrap-around shields. For example, Kimock, et al., U.S. Pat. Nos. 5,135,808 and 5,190,807 disclose a direct ion beam deposition process for producing abrasion wear-resistant DLC coatings on products such as optical lenses and sunglass lenses.
For both AR coatings and colored mirror coatings, the dielectric layers are most commonly deposited by thermal evaporation, electron beam evaporation, ion beam assisted electron beam evaporation, or magnetron sputtering techniques which are well-known in the prior art.
While substantial improvements in environmental durability of these AR coatings and reflective colored mirror coatings have been made via the use of ion beam assisted deposition processes, AR coatings on plastic ophthalmic lenses and colored mirror coatings on plastic sunglass lenses remain very susceptible to damage by abrasion and scratching. Thus, improved coating methods are still required to produce dielectric coatings on plastic lenses which have excellent durability and resistance to abrasion and scratching.
Plastic ophthalmic lenses are typically made from materials such as polycarbonate, CR-39 poly(allyl diglycolcarbonate) or other acrylics, or "high index" plastics which can be olefins, urethanes, or copolymers of acrylics and other polymers. Plastic sunwear lenses are typically made from either poly(allyl diglycolcarbonate), acrylics, or polycarbonate. These plastic lenses are typically coated with acrylic, urethane, or polysiloxane dip coatings or spin coatings. Although these coatings significantly improve the abrasion resistance of the lenses compared to uncoated plastic, the abrasion-resistance of coated plastic lenses is still inferior to that of glass lenses.
Because of its light weight, superior impact resistance, and high refractive index, polycarbonate is often the material of choice for fabrication of ophthalmic lenses, sunglass lenses, and sports optics. Of all the optical plastics, polycarbonate is also the most easily damaged by scratching and abrasion. Therefore, AR coatings and colored mirror coatings on polycarbonate lenses are also highly subject to damage by scratching and abrasion.
In coatings applied to plastic lenses, flexibility or elasticity is desirable so that crazing does not occur during flexure or bending of the substrate. In addition, with increased coating flexibility, degradation in the impact resistance of the lens is avoided, and deep scratches are less noticeable. Flexibility, or conversely brittle behavior can be quantified by stretching or bending a sample with the coating on the convex surface and measuring the percent elongation (100.DELTA.L/L) at which the coating fails, i.e., develops fine cracks. This percent elongation will be referred to herein as the strain to microcracking.